This invention relates to a gas detector-alarm employing a hot-wire gas sensor and more particularly to a technology to reduce electric power to heat the heat wire.
With gas detectors that determine gas concentration by means of a gas sensor with a hot wire, most of electric power supplied thereto is consumed by the gas sensor. As such, provisions are made to supply electricity to the hot wire at given intervals for the shortest possible but long enough time to permit gas detection, as proposed in Japanese Provisional Utility Model Publication No. 14595 of 1987, and, in addition to the above, to ensure continued supply of electricity when the presence of gas is detected, as proposed in Japanese Provisional Patent Publication No. 233699 of 1991.
To detect transient changes in gas concentration after the occurrence of gas leakage by these methods, however, heating time must be increased, with a resulting increase in power consumption during the time when gas is present.
The latter of the above two methods, although capable of detecting transient changes in gas concentration, involves the problem that power consumption increases when the concentration of gas present exceeds a certain level.
A gas detector-alarm according to this invention determines gas concentration by means of a hot-wire gas sensor to which pulsed actuating current is supplied. Said gas detector-alarm has an actuating-current control unit that supplies said pulsed actuating current with a cycle T0 when the concentration of gas detected by said hot-wire gas sensor is lower than a first reference level and with a cycle T1, which is shorter than said cycle T0, at least once when the concentration is higher than a second reference level.
When gas concentration exceeds the first reference level, the current-supply cycle is reduced to T0 in preparation for further concentration increase. When gas concentration is found to exceed the second reference level and reaches the warning level, the current-supply cycle is increased to T1 to save electricity in the presence of high-concentration gas.
The object of this invention is to provide a gas detector-alarm employing a hot-wire gas sensor that triggers an alarm without failure within a predetermined time while keeping to a minimum power consumption after gas concentration has exceeded a given level.